In the airlay web forming process in use by E.I. du Pont de Nemours and Company (DuPont) in the manufacture of spunlaced fabrics sold under the trademark Sontara.RTM., fiber is carried by a relatively fast moving air stream to a screen conveyor forming a web of randomly arranged fibers. The commercial process is disclosed and described in U.S. Pat. No. 3,797,074 to Zafiroglu. While the Zafiroglu arrangement has been in successful use for a number of years, the webs formed thereby are generally not uniform, and the edges are often completely unacceptable. At the edges, as much as six to eight inches at both sides must be separated and removed from the web because of the irregularities and defects which will lead to defects in the final product. Typically, the edge portions of the fiber are vacuumed away to render relatively clean cut edges of the batt. While the fiber is recovered to be subsequently reformed into the web, the inability to utilize the full width of the manufacturing capability has reduced the productivity of the system.
Upon investigation, it has been hypothesized that the air flow which carries the fiber to the screen conveyor has vortices or turbulence at the peripheral sides which renders the unsatisfactory product. In accordance with Zafiroglu, the air that is used to carry the fiber is introduced through a system of large conduits and fans. Prior to receiving the fiber, the air flow is directed through screens and straighteners to provide a uniform flow substantially free of large-scale turbulence and vortices. Thereafter, the large volume, relatively slow moving air flow is accelerated through a converging section or nozzle into a reduced cross sectional area conduit which is substantially flat and wide to be suited for laying down a wide web. It is believed that the Zafiroglu designed acceleration nozzle creates, or allows the creation of, the vortices and turbulence at the peripheral sides which is believed responsible for the edge defects.
Accordingly, it is an object of the present invention to provide an airlay web former arrangement which substantially reduces the edge defects of the web and overcomes the drawbacks of the present arrangements as described above.
It is a more particular object of the present invention to provide an arrangement for accelerating an airstream for an airlay web former which provides a substantial improvement over present designs in avoiding the creation or development of large scale turbulence and vortices.
The above and other objects of the invention are achieved by the provision of an acceleration device which comprises a duct having a generally rectangular cross sectional inlet portion having generally flat straight portions at all of the top, bottom and sides thereof and a generally rectangular cross sectional outlet portion having generally flat straight portions at all of the top, bottom and sides thereof. The cross sectional area of the outlet portion is smaller than the cross sectional area of the inlet portion. The device further comprises an acceleration portion between the inlet and outlet portions wherein all of the top, bottom and sides converge inwardly from the cross sectional shaped inlet portion to the cross sectional shaped outlet portion.
The invention may also be characterized by the converging portions of the device having a shape wherein the converging portions have a continuously differentiable curvature.